Q.(a) Why can formaldehyde not be prepared using the Rosenmund reduction reaction?

Rosenmund reduction converts an acid chloride to an aldehyde using $H_2$ over a poisoned Pd-BaSO4 catalyst.

Rosenmund's reduction is the controlled catalytic hydrogenation of an acyl (acid) chloride to an aldehyde:

$R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl$

The catalyst used is palladium deposited on barium sulphate, and crucially it is 'poisoned' (partially deactivated) — usually with a small amount of sulphur or quinoline (Lindlar-type deactivation). This poisoning is essential because it prevents the catalyst from being active enough to further reduce the newly-formed aldehyde all the way down to a primary alcohol; the reaction is thereby halted cleanly at the aldehyde stage.

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Selective reduction of an acid chloride to an aldehyde using $H_2$ over a poisoned (partially deactivated) Pd–BaSO$_4$ catalyst is called the Rosenmund reduction.

$C_6H_5COCl \xrightarrow[Pd - BaSO_4\ (\text{poisoned with S/quinoline})]{H_2} C_6H_5CHO + HCl$

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Acid chloride + H2 over Pd/BaSO4 -> aldehyde: this is the Rosenmund reduction.

In the Rosenmund reduction, an acyl chloride is reduced to an aldehyde by hydrogen gas over a palladium catalyst supported on barium sulphate:

R-COCl + H2 --(Pd/BaSO4)--> R-CHO + HCl

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Rosenmund reduction (acid chloride + H2, Pd/BaSO4 poisoned catalyst) stops at the aldehyde stage; it is a method for preparing aldehydes, not ketones.

• Oxidation of a secondary alcohol (with, e.g., PCC or acidified K2Cr2O7): gives a ketone — a valid ketone-preparation method.
• Catalytic dehydrogenation of a secondary alcohol (passing vapours over hot Cu): gives a ketone — a valid method.
• Dry distillation of the calcium salt of a fatty acid (Ca salt of a carboxylic acid, heated): gives a ketone (e.g., calcium acetate → acetone) — a valid method. …

Benzoyl chloride is selectively reduced by H2 over a Pd/BaSO4 (poisoned/partially deactivated palladium) catalyst to give benzaldehyde — this is the Rosenmund reduction.

The scheme shows benzoyl chloride (C6H5–COCl) reacting with hydrogen gas in the presence of a palladium catalyst supported on barium sulphate (Pd/BaSO4), which is partially poisoned (deactivated) so that the reduction stops at the aldehyde stage instead of going further to the alcohol.

C6H5–COCl + H2 --(Pd/BaSO4)--> C6H5–CHO + HCl

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Catalytic hydrogenation of an acyl chloride over poisoned palladium (Pd/BaSO4) is the Rosenmund reduction, giving an aldehyde.

R-CO-Cl + H2 --(Pd on BaSO4)--> R-CHO + HCl

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This is the Rosenmund reduction: an acid chloride is selectively reduced to an aldehyde using H2 over a poisoned Pd catalyst.

Acyl chlorides (acid chlorides) can be reduced to aldehydes by controlled hydrogenation over palladium supported on barium sulfate and partially poisoned with sulfur/quinoline. The poisoning is essential — it stops the catalyst from over-reducing the product all the way to the alcohol, so the reaction halts cleanly at the aldehyde stage.

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Benzoic acid is first converted to its acid chloride with $PCl_5$, and that acid chloride is then selectively reduced back to the aldehyde stage by Rosenmund reduction (catalytic hydrogenation over a poisoned palladium catalyst).

Step 1 — reaction with $PCl_5$: $PCl_5$ replaces the $-OH$ of the carboxylic acid with $-Cl$, converting the acid to its acyl (acid) chloride, along with $POCl_3$ and $HCl$ as by-products:

$C_6H_5COOH + PCl_5 \rightarrow C_6H_5COCl\;(=C) + POCl_3 + HCl$

$C$ is benzoyl chloride.

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Controlled catalytic hydrogenation of an acid chloride over a poisoned palladium catalyst (Rosenmund reduction) stops at the aldehyde stage, without over-reduction to the alcohol.

Benzoyl chloride, $C_6H_5\text{--COCl}$, when treated with hydrogen gas in the presence of palladium supported on barium sulphate (partially poisoned/deactivated, often with sulphur or quinoline, so that the catalyst is not active enough to reduce the aldehyde further to the alcohol), undergoes selective reduction of the acid chloride group to an aldehyde. This is the Rosenmund reduction:

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The Rosenmund reduction uses hydrogen gas over a specially 'poisoned' palladium catalyst to reduce an acid chloride only as far as the aldehyde, without over-reducing it to the alcohol.

An acid chloride (R-COCl) can, in principle, be hydrogenated all the way to a primary alcohol. To stop the reduction cleanly at the aldehyde stage, the reaction is run with H2 gas over palladium supported on barium sulphate, whose activity has been deliberately reduced ('poisoned') using sulphur or quinoline. This partial-deactivation prevents the aldehyde product from being further reduced:

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Acid chloride → aldehyde over Pd/BaSO₄ = Rosenmund reduction.

Rosenmund reduction converts an acyl chloride to an aldehyde using H₂ over palladium supported on barium sulfate (the catalyst is 'poisoned' with BaSO₄/quinoline to stop over-re …